Gypsum wallboard is manufactured in what might be thought of as two steps or operations. Wallboard production is generally described in U.S. Pat. Nos. 3,592,670 and 3,389,042. First, there is manufactured a paper cover sheet, generally a multi-ply sheet manufactured on a cylinder machine in which a cylindrical screen rotates in a vat containing an aqueous slurry of paper furnish. Conventional sizing compounds such as rosin and alum are added to the selected vat to properly size some or all plies. Various numbers of plies after being formed on the screen, are removed therefrom and are superimposed to form an essentially unitary piece of paper, typically of about 0.020 inches thick.
In the gypsum board plant, as a second step, an aqueous slurry of settable gypsum plaster, having more than sufficient water for hydration and setting of the gypsum, is spread on the advancing paper cover sheet and a second paper cover sheet is continuously applied on top of the gypsum as the onset, wet board is moved through the forming apparatus. The partially hardened, endless board is then cut into desired lengths and passed into a high temperature drying kiln.
The paper ply of the multi-ply cover sheet directly adjacent the plaster core is known as the bond ply or core-side ply. This ply, and the filler plies intermediate to the bond ply and the outermost ply or top liner ply, are commonly formed from a mixture of waste paper pulps. The bond ply, top liner ply and filler plies may be made of substantially the same or different paper furnish. For example, the bond ply and filler plies formed from a mixture of waste paper pulps can be employed in the manufacture of the paper cover sheet. Where an attractive outermost ply is required, such as creamface, it may be made from groundwood and sulfite or other type of suitable pulp.
Generally, the paper cover sheets used in gypsum wallboard range in thickness from about 0.010 to 0.030 inch, and preferably 0.016 to 0.023 inch, and have tensile strengths in the machine direction of about 50-115 lbs./in. and in the across-machine direction within the range of about 15-40 lbs/in.
It is necessary to have a strong bond between the gypsum core and the paper cover sheet in the finished board to prevent separation or delamination of the paper from the core. Separation can occur either in the form of "film peeling" in which a thin layer of the core is removed along with the paper and adheres thereto, or a "clean peel" in which the paper is cleanly separated from the gypsum core.
Generally, it has been considered necessary to provide for absorption of water from the gypsum slurry into at least the bond ply of the multi-ply paper covering sheet. The water absorption mechanism carries some dissolved gypsum into the paper where the gypsum crystallizes and provides a resultant mechanical linking of the paper to the gypsum core. This absorption of water by the paper, however, decreases the ratio of water to unset gypsum in a very thin layer of the gypsum core immediately adjacent the paper cover sheet, particularly during initial setting of the gypsum core when the ratio of water to gypsum is critical to obtain the desired quality of set gypsum throughout the core. In the thin layer of unset gypsum having a relatively lower water ratio, a different crystal growth occurs from that throughout the balance of the core, creating what is termed stratification. Stratification becomes increasingly apparent with efforts to accelerate the setting of the gypsum, or to decrease the drying time in the kiln. Also, gypsum manufactured from ores of relatively low purity may aggravate the problem.
As the degree of stratification increases, the tendency of the thin layer of gypsum to become recalcined during drying in the kiln increases. Accordingly, stratification creates a limiting factor in the drying speed and thus in the overall production rate for a manufacturing facility. If the two stratified layers of gypsum adjacent the two paper cover sheets become recalcined during drying, the integrity and strength of the core at these layers is reduced permitting the cover sheets to be susceptible to so-called "film peeling" which is manifested by separation of the paper from the core with a thin layer of the core adhering to the paper.
The tendency toward recalcination of the stratified layers of gypsum can be lessened, for example, by adding a small percentage of starch, or other suitable water retention agent, to the gypsum mix used to form the core. The starch migrates toward, and partially into, the paper during the drying and, by its hydrophilic nature, tends to retain water wherever the starch becomes concentrated, including generally the interface zone where the stratification occurs. The greater portion of the starch, however, migrates into the paper because of the absorptiveness of the paper. Where a loss of starch from the gypsum core to the paper occurs, another type of paper core separation, known as "clean peel," occurs wherein the paper cleanly separates from the core with substantially no core particles clinging to it. For sufficient protection against recalcination in the zone of stratification and to compensate for starch loss, a larger amount of starch is generally necessary than would be necessary if the starch could be concentrated in the outermost zone of the gypsum core.
The additional amounts of starch required in a core formulation, as above stated, and the absorptivity of the paper cover sheet necessitate greater portions of water in forming the core, thus increasing the amount of drying ultimately required and deleteriously affecting the rate at which the board can be manufactured with given drying facilities.
In addition to the problems of separation or delamination at the interface between the gypsum core and the cover sheets, the multi-ply paper cover sheets are also subject to intra-ply separation and/or delamination of each of the plies from the other during the board-drying operation. Various techniques have been utilized to obviate delamination of the plies, such as the inclusion of wet strength resins in the paper cover sheet. Aminoplastic resins such as melamine-formaldehyde or urea-formaldehyde resins are commonly employed, and the resins are preferably added during the paper-making stage of the gypsum board manufacturing operation. While the use of aminoplastic resins for imparting wet strength to the paper plies minimizes delamination of the paper plies, this is an added expense in the cost of manufacture.
In further investigations of the problems of stratification, recalcination and delamination, the face of the paper cover sheet in contact with the gypsum core was treated with a suitable material which would make it highly repellent to water, or more specifically non-absorptive, without substantially decreasing the normal porosity of the paper. Suitable materials which effect this objective and treating techniques are described in U.S. Pat. No. 3,307,987 by David Bieri.
At various points in the gypsum wallboard manufacturing process, materials are added for different purposes to gypsum stucco during the mixing of the feed slurry.
Starch, dispersant, accelerators, foam and water are some of the materials fed into the mixer with the stucco in order to make up the slurry for the wallboard formation. Other special materials are added if fire resistance and other characteristics are required of the wallboard.
Starch is used to protect the gypsum crystals that form the bond with the paper. These crystals, located at the edges of the wallboard, are subject to dehydration during the period of high temperatures in the drying kilns. If dehydration occurs, the paper will peel off the board core. The starch holds water during kiln drying and protects these bonding crystals from severe dehydration.
An accelerator is used as the catalyst to control the time that is desired to "set" the board on the production line. The minimum quantity of accelerator is employed which gives the hardening rate needed for the speed and equipment of a production line. An excess of accelerator can cause a weakened board core.
Water reducing agents are used to minimize the excess water required for processability of the stucco slurry. They also reduce surface tension and allow the slurry to flow with less frictional resistance. Foam is added in order to give about 25% by volume of soap bubbles to the slurry. This provides the board with the lighter weight necessary for easy handling, allows the board to be nailed without cracking or breaking, and gives some flexibility to the wallboard. Pulped paper is added to furnish a web or fibers to help hold the crystalline core together and also add flexibility to the finished product. Fiberglass has the same purpose in the fire rated wallboard products. Water is needed to make a slurry out of all the ingredients so that they will form a homogeneous mixture. The water also furnishes one ingredient needed for the reaction of stucco with water to make the gypsum wallboard. Therefore, water carries the ingredients and furnishes the medium in which the wallboard forming takes place. The ratio of water to stucco is usually 9 to 10. In the production of wall board on a commercial scale board forming machine and in particular the formation of the core portion, a part of the total water volume to be used in the slurry which will form the core is added to an accelerator tube along with an excess of fine gypsum stucco particles to form the aqueous suspension wherein the stucco is hydrated and forms fine calcium sulfate dihydrate crystals. In some cases where a lighter weight wallboard is desired another part of the total water content of the slurry is added to a foam generator with an aqueous solution of detergent or soap and agitated therein to form a foam. The remainder of the slurry water content is charged into a beater along with starch, paper stock, a retarder and other additives. The mixture is agitated by the beater blades for approximately one hour. All of the components of the core formulation are then brought together in a pin mixer where they are agitated briefly to form a uniform slurry which is discharged at a controlled rate onto a sheet of wallboard face paper continuously advancing on a moving conveyor whereon the wallboard is formed. A second sheet of face paper is then disposed on top of the continuously advancing layer of slurry. The core is allowed to harden sufficiently so that the wallboard may be handled without being damaged and it is then placed on a conveyor which carries it through drying ovens where substantially all of the free water is removed. The slurry is discharged from the mixture onto a continuously moving paper covered line. This line normally runs at a speed of about 125 to 150 feet per minute. The paper is formed into an envelope which holds the stucco slurry until it has a chance to react and bond itself intimately to the paper. It then hardens, or sets, and then it can be cut and mechanically positioned for drying. Usually, 75% of the water added during forming -must be evaporated out of the board during drying (about 1 pound per square foot of wallboard). This drying time is usually 45 to 60 minutes and requires 2 to 2.5 million BTU per one thousand square feet of regular 1/2 inch thick wallboard. To increase energy efficiency in manufacturing processes, it is advantageous to decrease on the water required to manufacture gypsum wallboard, and thus, cut down on the energy required to dry the finished product. Upon completion of the drying process, the board is bundled, taped and stacked for shipment.
In order to make the process more efficient, it his become necessary to modify some of the standard additives. For example, reformulation of the dispersant is required in order to make it stronger acting so that the amount of water required may be reduced, while overcoming the properties causing soft, crumbly board and lengthened setting and drying time.
A dispersant formulation of a calcium lignin sulfonate and an ammonium linear alcohol ether sulfate for wallboard production is described in U.S. Pat. No. 4,222,984. Naphthalene condensate polymers may also be useful. Most dispersants presently used by the industry are by-products from the pulp digesting process, wherein the lignin is dissolved from the wood chips allowing wood fibers to separate. The lignin is dissolved by sulfonic acid and the lignin sulfonate by-product is separated, recovered and sold to many industries as a dispersant. These lignin sulfonate by-products contain other chemicals from the pulping process, especially fairly large amounts of wood sugars and smaller amounts of soluble chemicals. In most tests made, the lignin sulfonates, especially the sodium salts, were harmful to the quality of the finished gypsum test specimens. They are especially undesirable used in large amounts because of the possibility of humidified splits occurring after the wallboard is shipped.
One object of the present invention was to develop an economical dispersant formulation which would not have the deleterious effect on wallboard quality that the lignins showed at higher use levels. Another object of this invention was to develop a dispersant which would increase the diffusion rate of water vapor from the wallboard during the oven drying process, and thereby increase productivity and fuel efficiency.